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Message-ID: <127f992b-1c53-49e1-8939-65b1d086fac5@t-8ch.de>
Date: Tue, 4 Feb 2025 11:08:11 +0100
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
Danilo Krummrich <dakr@...nel.org>, Lyude Paul <lyude@...hat.com>,
Alexander Lobakin <aleksander.lobakin@...el.com>, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Bjorn Helgaas <bhelgaas@...gle.com>, Jonathan Cameron <Jonathan.Cameron@...wei.com>,
Liam Girdwood <lgirdwood@...il.com>, Lukas Wunner <lukas@...ner.de>, Mark Brown <broonie@...nel.org>,
Maíra Canal <mairacanal@...eup.net>, Robin Murphy <robin.murphy@....com>,
Simona Vetter <simona.vetter@...ll.ch>, Zijun Hu <quic_zijuhu@...cinc.com>, linux-kernel@...r.kernel.org,
linux-usb@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 1/3] driver core: add a faux bus for use when a simple
device/bus is needed
On 2025-02-03 15:25:17+0100, Greg Kroah-Hartman wrote:
> Many drivers abuse the platform driver/bus system as it provides a
> simple way to create and bind a device to a driver-specific set of
> probe/release functions. Instead of doing that, and wasting all of the
> memory associated with a platform device, here is a "faux" bus that
> can be used instead.
>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
> drivers/base/Makefile | 2 +-
> drivers/base/base.h | 1 +
> drivers/base/faux.c | 189 ++++++++++++++++++++++++++++++++++++
> drivers/base/init.c | 1 +
> include/linux/device/faux.h | 33 +++++++
> 5 files changed, 225 insertions(+), 1 deletion(-)
> create mode 100644 drivers/base/faux.c
> create mode 100644 include/linux/device/faux.h
<snip>
> diff --git a/drivers/base/faux.c b/drivers/base/faux.c
> new file mode 100644
> index 000000000000..0eba89a5cd57
> --- /dev/null
> +++ b/drivers/base/faux.c
> @@ -0,0 +1,189 @@
> +// SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (c) 2025 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> + * Copyright (c) 2025 The Linux Foundation
> + *
> + * A "simple" faux bus that allows devices to be created and added
> + * automatically to it. Whenever you need a device that is not "real",
> + * use this interface instead of even thinking of using a platform device.
> + *
> + */
> +#include <linux/device/faux.h>
> +#include <linux/err.h>
> +#include <linux/init.h>
> +#include <linux/slab.h>
> +#include <linux/string.h>
> +#include "base.h"
> +
> +/*
> + * Internal wrapper structure so we can hold the memory
> + * for the driver and the name string of the faux device.
> + */
> +struct faux_object {
> + struct faux_device faux_dev;
> + struct device_driver driver;
> + const struct faux_driver_ops *faux_ops;
> + char name[];
> +};
> +#define to_faux_object(x) container_of_const(dev, struct faux_object, faux_dev.dev);
> +
> +static struct device faux_bus_root = {
> + .init_name = "faux_bus",
> +};
> +
> +static int faux_match(struct device *dev, const struct device_driver *drv)
> +{
> + struct faux_object *faux_obj = to_faux_object(dev);
> +
> + /* Match is simple, strcmp()! */
> + return (strcmp(faux_obj->name, drv->name) == 0);
> +}
> +
> +static int faux_probe(struct device *dev)
> +{
> + struct faux_object *faux_obj = to_faux_object(dev);
> + struct faux_device *faux_dev = &faux_obj->faux_dev;
> + const struct faux_driver_ops *faux_ops = faux_obj->faux_ops;
> + int ret = 0;
> +
> + if (faux_ops && faux_ops->probe)
> + ret = faux_ops->probe(faux_dev);
> +
> + return ret;
> +}
> +
> +static void faux_remove(struct device *dev)
> +{
> + struct faux_object *faux_obj = to_faux_object(dev);
> + struct faux_device *faux_dev = &faux_obj->faux_dev;
> + const struct faux_driver_ops *faux_ops = faux_obj->faux_ops;
> +
> + if (faux_ops && faux_ops->remove)
> + faux_ops->remove(faux_dev);
> +}
> +
> +static const struct bus_type faux_bus_type = {
> + .name = "faux_bus",
Is the _bus suffix intentional?
Other busses don't have it.
> + .match = faux_match,
> + .probe = faux_probe,
> + .remove = faux_remove,
> +};
> +
> +static void faux_device_release(struct device *dev)
> +{
> + struct faux_object *faux_obj = to_faux_object(dev);
> + struct device_driver *drv = &faux_obj->driver;
> +
> + /*
> + * Now that the device is going away, it has been unbound from the
> + * driver we created for it, so it is safe to unregister the driver from
> + * the system.
> + */
> + driver_unregister(drv);
> +
> + kfree(faux_obj);
> +}
> +
> +/**
> + * __faux_device_create - create and register a faux device and driver
> + * @name: name of the device and driver we are adding
> + * @faux_ops: struct faux_driver_ops that the new device will call back into, can be NULL
> + * @owner: module owner of the device/driver
> + *
> + * Create a new faux device and driver, both with the same name, and register
> + * them in the driver core properly. The probe() callback of @faux_ops will be
> + * called with the new device that is created for the caller to do something
> + * with.
> + */
> +struct faux_device *__faux_device_create(const char *name,
> + struct faux_driver_ops *faux_ops,
const
> + struct module *owner)
What about attributes?
> +{
> + struct device_driver *drv;
> + struct device *dev;
> + struct faux_object *faux_obj;
> + struct faux_device *faux_dev;
> + int ret;
> +
> + faux_obj = kzalloc(sizeof(*faux_obj) + strlen(name) + 1, GFP_KERNEL);
> + if (!faux_obj)
> + return NULL;
> +
> + /* Save off the name of the object into local memory */
> + strcpy(faux_obj->name, name);
> +
> + /* Initialize the driver portion and register it with the driver core */
> + faux_obj->faux_ops = faux_ops;
> + drv = &faux_obj->driver;
> +
> + drv->owner = owner;
> + drv->name = faux_obj->name;
Assuming most names are constant, this would be better with kstrdup_const().
Which is also used by dev_set_name() under the hood.
> + drv->bus = &faux_bus_type;
> + drv->probe_type = PROBE_PREFER_ASYNCHRONOUS;
> +
> + ret = driver_register(drv);
> + if (ret) {
> + pr_err("%s: driver_register for %s faux driver failed with %d\n",
> + __func__, name, ret);
> + kfree(faux_obj);
> + return NULL;
> + }
> +
> + /* Initialize the device portion and register it with the driver core */
> + faux_dev = &faux_obj->faux_dev;
> + dev = &faux_dev->dev;
> +
> + device_initialize(dev);
> + dev->release = faux_device_release;
> + dev->parent = &faux_bus_root;
> + dev->bus = &faux_bus_type;
> + dev_set_name(dev, "%s", name);
> +
> + ret = device_add(dev);
> + if (ret) {
> + pr_err("%s: device_add for %s faux device failed with %d\n",
> + __func__, name, ret);
> + put_device(dev);
> + return NULL;
> + }
> +
> + return faux_dev;
> +}
> +EXPORT_SYMBOL_GPL(__faux_device_create);
<snip>
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